Torbutton's ‘banned ports’ make it detectable when off
Torbutton leaves ports 8118, 8123, 9050, and 9051 in Firefox's ‘banned ports’ list when it is disabled. A web server can use this fact to detect Torbutton's presence, even in non-Tor mode, by telling a user's browser to connect to it on multiple ports, including those ‘banned’ by Torbutton.